@leon, I was able to debug your model and found that there is indeed some noise in your normal map. Below you can see the normal map containing the hood of the car placed on a sphere. There is only a base color factor set to red and a roughness factor set to 0 on the asset. I rotated the environment lighting around in the scene, and you can see all of the noise that you pointed out in your question above:
I then pulled your map into photoshop and ran the cursor around what looked to be a flat area watching the info window. The colors were hopping by one or two in R, G, or B pretty consistently, which was my first clue. Then I dropped a solid chip of 127, 127, 255 with a black stroke around it and added a curve adjustment layer on top of everything. I modified the curve as you can see in the image below to maximize the contrast in any noise in the map:
You can see that There is a lot of change of tone in your normal map when compared with the flat blue in the chip. Every slight difference in the normal map means a different normal vector on that pixel. With enough changes in the normal vector across a field of pixels, you will see this kind of ripple in the surface.
You mentioned that your original textures were done in Painter so I would go back into the layers of the file and see where you may have introduced some noise in the material. My guess is that if a smart material was used, there nay be some height information baked into the normal to give this orange peel effect. Granted, if you are using clear coat on top of this, you may want to keep this texture in the normal as it will give you some lighting information on the paint below the clear coat.
I used this same approach when making our demo for clear coat where I applied clear coat to a carbon fiber material and backed off the clear coat intensity to 0.9 until you could just see some lighting information on the carbon fiber. This can be very effective at giving depth to your assets:
Let me know if this doesn’t solve your issue or if you have more questions. Take care!